a+1/a=3
（a+1/a）^2=9
a^2+1/a^2+2=9
a^2+1/a^2=7
a^2/(a^4+a^2+1) 分子、分母同时除以a^2
=1/(a^2+1+1/a^2)
=1/(7+1)
=1/8